Many circuits today use voltage controlled oscillators (VCOs) to generate a reliable periodic signal. For example, phase locked loops (PLLs) typically include a VCO for duty cycle correction, phase/delay compensation, and/or frequency multiplication. Although VCOs come in various configurations (e.g., a ring oscillator, a crystal oscillator, or an LC tank circuit), a VCO generally receives a voltage tuning signal indicative of a desired periodic output signal. The applied voltage signal may be varied to adjust, or tune, the frequency of the output signal. For example, using such a voltage signal, a PLL may tune a VCO until the VCO outputs a desired waveform. If, at any point of operation, the waveform needs to be adjusted or corrected, the PLL will adjust the magnitude of the voltage signal to tune the VCO.
Unfortunately, because VCOs are located in proximity to other circuitry, often sharing the same substrate and biasing, VCOs are subjected to noise (e.g., common-mode voltage shifts). Moreover, environmental factors, such as temperature and electromagnetic radiation, may also produce noise. Such noise reduces the performance of a VCO and may increase operational overhead. For example, common-mode voltage noise within a VCO may increase the capacitance of circuit elements that the VCO comprises. When the VCO receives a tuning signal, the increased capacitance may increase the amount of time it takes the VCO to produce the correct waveform. Such a lag in time may likewise reduce the performance of circuits and devices that use the VCO. Therefore, it is desirable to provide a VCO that may be subjected to noise without comprising performance.